NCR 7458 - Manual (Page 24)

2-6 Chapter 2: POST Diagnostics
Terminal POST Errors
There are several POST routines that issue a POST Terminal Error and
shut down the system if they fail. Before shutting down the system, the
terminal-error handler issues a beep code signifying the test point
error, then writes the error to port 80h and attempts to initialize the
video. The handler writes the error in the upper left corner of the
screen (using both mono and color adapters). The routine derives the
beep code from the test point error as follows:
1. The 8-bit error code is broken down to four 2-bit groups.
2. Each group is made one-based (1 through 4) by adding 1.
3. Short beeps are generated for the number of times in each group.
Example: Test point 01Ah = 00 01 10 10 = 1-2-3-3 beeps
Test Points and Beep Codes
At the beginning of each POST routine, the BIOS outputs the test point
error code to I/O address 80h. Use this code during troubleshooting to
establish at what point the system failed and what routine was being
performed.
If the BIOS detects a terminal error condition, it halts POST after
issuing a terminal error beep code (see previous section) and
attempting to display the error code on upper left corner of the screen
and on the port 80h LED display.
If the system hangs before the BIOS can process the error, the value
displayed at the port 80h is the last test performed. In this case, the
screen does not display the error code.